| I have been troubleshooting rough idle when cold on my '91 300zx and am having trouble determining if my AAC solenoid is bad or not. Things that I have learned and tested:
-The AAC is connected to battery voltage with ignition on and the ground wire is controlled by a PWM signal from the ECU via pin 4. -I have verified proper voltage between the positive pin at the AAC connector and battery ground.
-I have verified continuity between AAC connector and pin 4 of the ECU harness connector.
-I have verified that there are no shorts to ground on the pwm wire going to the AAC.
-I have verified proper resistance on the AAC solenoid and get an audible click when 12 volts is connected to solenoid. At this point I couldn't see any reason why it wouldnt work. I even blew into the IACV with my mouth while connecting the AAC solenoid to 12v and felt the solenoid open up allowing me to blow through it easier. The problem is I cannot get any change in rpm when changing the duty cycle using data scan(any value between 1 and 100 has no affect on idle rpm). Also there is no change in rpm when disconnecting the yellow AAC valve connector. I originally thought that my Z1 throttle bodies were the issue. The butterfly valve in the throttle body does not seal well against the inner bore so they allow too much air into the engine when at idle. I thought that if they let enough air to pass then it would render the IACV useless since the amount of air the IACV controls is much smaller then the amount of air leaking pass the tb's: [ http://twinturbo.net/nissan/300zx/forums/general/view/2495647/Poor-sealing-Z1-tbs-and-how-they-effect-idle-control.html ] Like I mentioned in the above thread, I plugged the throttle bodies and was able to get the rpm's down to a normal level but was still unable to get any change in rpm by manipulating the duty cycle in data scan or by disconnecting the AAC solenoid. But today I connected 12 volts directly to the AAC valve when the engine was running and the rpms actually did rise about 200 or so. I suspect that maybe the ecu itself is the problem...is that possible? Maybe a bad transistor or something that controls the pwm signal. Wish I had an oscilloscope. Curious what you think.
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